Some of the disease states that pose the greatest danger to vision are thought to involve abnormalities in the oxygen supply to the retina or the oxygen metabolism. These include diseases such as diabetic retinopathy, which is one of the most common causes of blindness in the world and retinopathy of prematurity, retinal vein and artery occlusions. It has also been suggested that ocular blood flow is compromised in glaucoma, including normal tension glaucoma, and that the loss of optic nerve fibers in glaucoma patients may be due to ischemia.
The principle of spectrophotometric hemoglobin oximetry is old and the method has been developed into clinically useful instrumentation in several specialties of medicine. The finger or earlobe oximeter, commonly used in anesthesiology and intensive care is an example of the successful application of oximetry in medicine.
Information on retinal and optic nerve oxygenation in health and disease is mostly derived from animal research. Non-invasive measurement of oxygenation in the human retina and optic nerve has proved difficult but considerable progress has been made, particularly in recent years as digital technology has evolved. The potential use of retinal oximetry covers a range of areas, including assessment of oxygen metabolism in disease, the efficacy of treatment of these diseases in restoring or improving the metabolic conditions, either by laser treatment, surgery, lowering of intraocular pressure or by medication. Retinal oximetry may also be of use in elucidating further the physiological processes involved in disease states, such as in glaucoma.
In the 1990s, an important version of retinal vessel oximetry was developed at the University of Virginia, by James Beach and James Tiedeman [1, 2]. Their version was based on a method proposed by Delori [3] with additional improvements. The main advantage of the Beach and Tiedeman method is the ability to obtain simultaneously two or more images of light reflectance at different wavelengths (called multi-spectral images) from the same fundus using a fundus camera. In this manner it is possible to record reflectance at both oxygen-sensitive and insensitive wavelengths from exactly the same area on the fundus, and at precisely the same time. This allows for precise quantification of the effects of oxygen binding on the light absorption spectra of hemoglobin. Oximetric measurements of this kind are achieved by a system whose main component consists of a modified fundus camera, although the internal xenon flash of the camera is still used as a light source, unmodified. Other components of the system are a beam splitter, a gray scale digital camera, possessing high quality linear performance, and a computer. The digital camera replaces the image acquisition mechanism of the fundus camera. Flashes from the fundus camera are synchronized with recordings by the digital camera electronically. The image thus obtained with the digital camera is then fed into a computer for further analysis. Between the fundus camera and the digital camera there are image-splitting optics, which can project several separate images at the digital camera detector, as illustrated by FIG. 1.
In each of these optical pathways, which are separated by a beam splitter and dichroic mirrors, there is a narrow band pass interference filter. Each filter has a 5 nm half-bandwidth and different central (peak) wavelengths of transmission. The filters, or rather their peak wavelengths, are chosen on the basis of the absorption spectra of oxyhemoglobin (HbO2) as before, and illustrated in FIG. 2. The filter bandwidth clearly affects measurements of the blood Optical Density (OD) and this can be evaluated by calculating the apparent light extinction coefficients of HbO2 and hemoglobulin with no oxygen (Hb) [2]. The extinction coefficients are nearly identical at the isosbestic wavelength of 586 nm, particularly at a narrow bandwidth, while considerably higher for Hb than HbO2 at the oxygen-sensitive wavelength (600 or 605 nm). Thus, the apparent Optical Density Ratio (ODR) between the oxygen-sensitive and isosbestic wavelengths reflects the oxygen saturation in retinal vessels and tissue in an inverse manner. With higher oxygen saturation the OD at 605 nm is lower.
OD is a measure of absorption; higher OD means greater absorption [5]. One factor that affects the optical density measured in fundus oximetry is the extravascular pigmentation of the fundus, which can vary greatly between subjects. Pigmentation may cause underestimation of ODs in vessels, in particular at 586 nm, because it causes stronger absorption of light. The effect of retinal pigmentation can be estimated by measuring the ratio of extravascular light reflection near vessels at the oxygen-sensitive and insensitive wavelengths used [2]. The extravascular light reflection ratio (EVR) is calculated as:
  EVR  =            I      586              η      ⁢                          ⁢              I        605            Where I586 is the intensity of 586 nm wave length light reflected, I605 is the intensity of 605 nm wave length light reflected, and η is a correction constant for light transmission through the instrument (evaluated separately), from the light source to the digital camera, via the beam splitter and optics. Experimentally it has been found that EVR is greater with higher fundus pigmentation [2]. In order to calculate vessel OD it is therefore necessary to correct for pigmentation by measuring intensity at both wavelengths in non-vascular areas.
One of the limitations of prior art retinal oximetry [13] includes difficulties in obtaining calibrated measures of actual oxygen saturation. An indirect calibration of the ODR, as it relates to actual oxygen saturation of vascular blood can be obtained by comparing the ODR and systemic O2 saturation, the latter controlled by having subjects inhale air mixtures of different composition, with 100% oxygen and lower. The reliability of this approach can be further enhanced by measuring the systemic O2 saturation with a finger pulse oximeter. It has been found experimentally in this manner that there is a fairly good inverse correlation between systemic O2 saturation and the ODR (lower ODR with higher systemic O2) in retinal arterioles [2], and venules [4] in normal human volunteers.
Clearly, examination of images of the fundus is an important diagnostic tool in ophthalmology. Fundus images allow inspection of arteries and veins that are usually hidden beneath the skin. Consequently, fundus images may be used to diagnose many diseases that affect the vascular structure.
The problems with the prior art methods used in retinal oximetry is the need for an expert to perform the oximetry, the high variability of the results, and poor reproducibility. The prior art methods are time consuming, and manual registration of the spectral images is inaccurate because of the error prone manual selection of sample points.